Measuring performance throughout construction can help to identify major performance problems as quickly as possible, which is almost always beneficial.
If a major problem becomes apparent, then some immediate redesign will likely be necessary.
If no major problem is apparent, then, if needed, minor optimizations can be performed later, when construction is nearly complete.
(Many would argue that worrying about minor optimizations during construction is not advisable, since optimizations can sometimes make code more obscure.)

The excellent JConsole tool
comes with JSE 6, and can be used to manage and monitor a local or remote Java Virtual Machine.

The Visual VM tool is also
available with modern JDK's. It's full-featured, and very helpful for finding memory leaks.
Another tool that now comes with the JDK is Java Mission Control.

If the above tools aren't available, then you are stuck with more primitive means of measuring an application's
size in memory. They depend on your platform. For example:

NT: Task Manager, look for java.exe or javaw.exe

Unix: get the process id "ps -e | grep java", pass it to
pmap, as in "pmap -x 6598", and look for total KB entry in the Private column

As well, the JDK itself comes with basic profiling tools. For example: